Aqueous drilling fluid additive, composition and process

ABSTRACT

The invention relates to an aqueous drilling fluid composition, a filtration control agent for utilization in said aqueous drilling fluid, and a method of forming a filter cake on the wall of a well for the reduction of filtrate from said drilling fluid, which are effective at high temperatures and pressures, by utilization of a terpolymer of: (1) a monovalent alkali metal salt of acrylic acid; (2) an hydroxy alkyl acrylate; and (3) acrylamide.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of co-pendingapplication Ser. No. 946,227, filed Sept. 27, 1978, now abandonedentitled "Aqueous Drilling Fluid Additive, Composition and Process," andassigned to the same assignee as this application.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The invention relates to the utilization of a terpolymer of: (1) amonovalent alkali metal salt of acrylic acid; (2) an hydroxy alkylacrylate; and (3) acrylamide, to effectively reduce the filtrate loss ofan aqueous drilling fluid used in the drilling of a subterranean well.

2. DESCRIPTION OF THE PRIOR ART

It is generally agreed among those skilled in the art that a rotarysystem is a most acceptable form of drilling an oil or gas well. Thissystem depends upon the rotation of a column of drill pipe to the bottomof which is attached a multi-pronged drilling bit. The bit cuts into theearth causing the cuttings to accumulate as drilling continues. As aresult, a drilling fluid must be used to carry these cuttings to thesurface for removal, thus allowing the bit to continue functioning andthe bottom hole to be kept clean and free of cuttings at all times.Drilling systems other than the rotary system are sometimes employedduring drilling operations. Nevertheless, these systems still require adrilling fluid to remove the bore hole cuttings and to otherwise performfunctions related to drilling fluids.

Oil-producing formations are generally porous layers having varyingdegrees of permeability to the flow of fluids such as oil, water, orgas. Consequently, the rate of oil production is largely determined bythe rate of flow through these permeable formations which, in turn, isdependent upon the porosity or permeability of the sand or stonepresent. In drilling through such a porous layer, it is desirable toemploy a drilling mud having such characteristics that excessive amountsof liquids or solids are prevented from penetrating the porousformation. The ability of the drilling mud to prevent excessiveformation fluid penetration is called filtration control.

Materials that have been used in the past to control filtration rates ofaqueous drilling fluids by plugging, producing cakes, or similarmethods, have included materials such as pre-gelatinized starch, sodiumcarboxylmethylcellulose, sodium polyacrylates, and lignites. Each ofthese materials have certain limitations. For example, lignite becomesineffective in high salt concentrations.

Acrylic and methacrylic derivatives, such as those which areco-polymerized with hydrocarbon substituted styrenes, such as alphamethyl styrene, para methyl styrene, 2-4 dimethyl styrene, and the likehave been utilized in drilling fluids. For example, U.S. Pat. No.2,718,497, to Oldham, et al, teaches the use of relatively highmolecular weight polymers of these materials to control water losscharacteristics of aqueous muds and clay dispersions. Additionally, U.S.Pat. No. 2,650,905, to Fordyce, et al, teaches the use of water solublesulfonated polystyrene derivatives for filtration control in water-baseddrilling fluids.

The prior art has utilized acrylic acid derivatives as thickeners fornumerous commercial purposes, including utilization in drilling fluids.For example, U.S. Pat. No. 4,059,552, to Zweigle, et al, teaches the useof acrylamide-sodium acrylate or acrylic acid-substituted acrylates. Asimilar thickening material is disclosed in U.S. Pat. No. 4,037,035, toBlanc, et al, by utilization of an acrylamide-sodium acrylateconstituent with an acrylamide-acrylic acid alkanolamine. Similarly,copolymers of acrylamide and sodium acrylate and acrylate derivativesthereof formed by irradiation polymerization are utilized as thickeners,as disclosed in U.S. Pat. No. 3,926,756 to Restaino. U.S. Pat. No.3,897,404 to Korte, et al, teaches utilization as thickeners forprinting paste and the like of substituted acrylamide-acrylicacid-acrylate derivatives.

Hydrophilic gels derived from 2-hydroxyethyl methacrylate have beenfound to be useful in a number of medical applications as material forgel filtration, such as copolymers of acrylamide, acrylicester-2-hydroxyethyl methacrylate, as disclosed in U.S. Pat. No.3,948,841, to Dusek.

Acrylamide-sodium acrylate-2-hydroxyethyl acrylate crosslinked agents ofa comparatively high molecular weight are utilized as soil stabilizersas disclosed in U.S. Pat. No. 3,651,002, to Higashimura, et al.

Acrylic acid derivatives such as copolymers of acrylamide and sodiumacrylate derivaties have been frequently and commercially utilized asflocculants for drilling fluids, and are disclosed in U.S. Pat. Nos.3,558,545, and 3,472,325 to Lummus. Similarly, a copolymer derived fromacrylic acid and acrylamide is disclosed in U.S. Pat. No. 3,323,603 toLummus et al, as a flocculant for aqueous drilling fluids.

In the present invention, enhanced filtration control is obtained byincorporation into an aqueous drilling fluid of a filtration controlagent consisting essentially of a terpolymer of: (1) a monovalent alkalimetal salt of acrylic acid; (2) an hydroxy alkyl acrylate; and (3)acrylamide. Although it is not fully understood, it is believed that theterpolymer will produce a filter cake along the bore hole to maintaineffective filtration control during circulation of the drilling fluidwithin the well.

It is an object of this invention to provide a filter cake which issubstantially unaffected by comparatively high bore hole temperaturesand pressures.

It is a further object of this invention to provide a filtration controlagent which will not materially affect the viscosity of the utilizeddrilling fluid system.

It is a further object of this invention to provide a comparatively lowcost, easily prepared, filtration control agent for use in aqueousdrilling fluids.

It is a further object of this invention to provide a filtration controlagent for aqueous drilling fluids which is comparatively insensitive tosalt environments and concentrations within the aqueous system.

It is a further object of this invention to provide a filtration controlagent which may be utilized in highly weighted drilling fluids.

Other objects and advantages of the present invention will be easilyappreciated by those skilled in the art from a reading of thedescription, examples and claims which follow.

SUMMARY OF THE INVENTION

The present invention provides a filter cake-producing substance whichis effective at high temperatures and is comparatively insensitive tosalt environments and concentrations found in aqueous drilling fluids byutilization of a terpolymer of: (1) a monovalent alkali metal salt ofacrylic acid; (2) an hydroxy alkyl acrylate; and (3) acrylamide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preparation of the terpolymer of the present invention incorporatesthree monomeric materials, which are: (1) a monovalent alkali metal saltof acrylic acid; (2) an hydroxy alkyl acrylate; and (3) acrylamide.

Any monovalent alkali metal salt of acrylic acid, such as the sodium,potassium, or cesium salt, may be utilized as a starting monomericadditive for preparation of the present filtration control agent. Di-and tri-valent salts have been found to be comparatively unacceptablebecause of their inherent precipitation characteristic when incorporatedinto aqueous systems. Additionally, acrylic acid variatives, such asmethacrylic acid salts, are not acceptable for use in the practice ofthe present invention because their comparative solubility parametersare significantly less in aqueous systems than that of theaforementioned acrylic acid water soluble salts. Additionally, since theacrylic acid derivatives set forth above are comparatively highly watersoluble in aqueous environments, their copolymerization activity isenhanced during the preparation reaction for the filtration controlagent of the present invention.

Although any monovalent alkali metal salt of acrylic acid may besuccessfully utilized in the preparation of the filtration control agentof the present invention, the sodium salt of acrylic acid is preferred.

Varying amounts of monovalent alkali metal salt of acrylic acid, asselected, may be incorporated into the preparation of the terpolymerfiltration control agent. It has been found that an amount of theselected monovalent alkaline metal salt of acrylic acid between aminimum of about 5 mole percent of the total composition of allmonomeric materials and a maximum of about 62 mole percent may beutilized. When utilizing the sodium salt of acrylic acid, a preferredand satisfactory terpolymer filtration control agent may be prepared byincorporating therein about 23 mole percent of sodium acrylate.

The terpolymer of the present invention also utilizes as an initialconstituent an hydroxy alkyl acrylate which is incorporated in order toprovide hydrogen bonding between chains incorporating a carboxyl groupand to provide a bulkier polymer end product. Additionally, theutilization of an hydroxy alkyl acrylate also enhances the viscosity ofthe terpolymer and renders it less sensitive to adverse effects of heavysalt concentrations, such as those found in aqueous brine solutionsencountered as a base constituent of an aqueous drilling fluid. Also, byincorporating the hydroxy alkyl acrylate material into the terpolymer,good hydrogen-bonded crosslinking can be achieved between the chainswhich will aid in resisting salt sensitivity of the resultant terpolymermaterial.

Numerous hydroxy alkyl acrylates may be utilized, such as 2-hydroxyalkyl acrylates, and 3-hydroxy alkyl acrylates, such as2-hydroxyethylacrylate, 2-hydroxybutylacrylate or, preferably,2-hydroxypropylacrylate.

The amount of the selected hydroxy alkyl acrylate for utilization in thecomposition of the present invention will vary, but a satisfactory endproduct may be made by incorporating therein a minimum of about 2.1 molepercent of hydroxy alkyl acrylate. A satisfactory end product also maybe produced by utilizing up to a maximum of about 40 mole percent ofhydroxy alkyl acrylate. Preferably, when 2-hydroxypropylacrylate isselected as the hydroxy alkyl acrylate, it has been found that about 4.0mole percent of 2-hydroxypropylacrylate may be utilized.

The third monomer for utilization in the terpolymer composition for usein the present invention is acrylamide, which has been found to have acertain electrolyte reaction when incorporated into the terpolymerutilized in the present invention. Although not fully understood, it isbelieved that the acrylamide monomer when polymerized with the otherconstituents of the terpolymer will act as a dispersant of carboxylgroups on the resultant polymeric chain. Although methacrylamide may besubstituted for the acrylamide monomer of the present invention, it hasbeen found that it is less tolerant of the heavier concentrations ofsalt solutions and environments and is less water soluble than theacrylamide. Therefore, the acrylamide monomer has been found to be moreacceptable for use in general in the present invention.

In our testing, we have found that a minimum of acrylamide monomer ofabout 31 mole percent may be incorporated into the terpolymer to obtaina satisfactory end product. A maximum amount of about 91 mole percent ofacrylamide may also be incorporated to achieve a satisfactoryterpolymer. However, we prefer to utilize about 73 mole percent ofacrylamide in the preparation of a preferred composition for use in thepresent invention.

The terpolymer of the present invention may be prepared using a varietyof known techniques. For example, emulsion, suspension or bulkpolymerization techniques may be utilized.

The terpolymer is further characterized in that a 0.2% by weight aqueoussolution of said terpolymer has a viscosity of about 25° C. of frombetween about 6 centipoises and about 85 centipoises, as determined at ashear rate of 511 reciprocal seconds.

As an effective filtration control agent, the terpolymer may be added toany aqueous base drilling fluid at the drilling or rig location in anamount from between about 0.25 p.p.b. to about 5 p.p.b. (pounds per 42gallon barrel). The amount needed will vary, of course, depending uponthe particular type of aqueous drilling fluid utilized, such as brine,sea water, or the like, the weight of the given drilling fluid, theclayey substances or clayey materials appearing therein, and thepresence and amount of other chemical additives, such as lignosulfonatedeflocculants, and the like. Simple and commercially available testingtechniques may be easily utilized at the well site to determine theamount of filtration control additive which must be added to thecirculatable drilling fluid to provide effective filtration control inthe subterranean well. Because of the loss of material in the well, suchas through adsorption onto the surface of the drilled solids and thelike, it may be necessary to incrementally add additions of theterpolymer to the drilling fluid from time to time to maintain therequired concentration.

A measure of the ability of the drilling fluid to form a thin imperviouscompressible filter cake may be determined by utilization of a simplefiltration test in which the filter cake is formed and pressed againstthe membrane of filter which is permeable to water. A standardizedprocedure for determining the filtration rate is described in "APIRecommended Practice RP 13 B Standard Procedure for Testing DrillingFluids," 2nd Edition (April 1969).

The preparation and use of the terpolymer to control filtrate in anaqueous drilling fluid is further described in the examples whichfollow:

EXAMPLE I.

A terpolymer of sodium acrylate-2-hydroxypropyl acrylate and acrylamidewas prepared utilizing an emulsion polymerization process. The moleratios of acrylic acid to acrylamide, mole percent of 2-hydroxypropyl,acrylate and molecular size were varied by controlling the reactionparameters. Six representative samples of the terpolymer were preparedwith the compositions of the various terpolymers being estimated basedupon their total nitrogen content. In the reaction, ammonium persulfatewas utilized as the polymerization initiator or catalyst. Reagent gradeacetone and reagent grade anhydrous methanol were utilized to form thesolvent. The emulsifier was sodium lauryl sulfate, and 2N aqueous sodiumhydroxide was utilized to neutralize the reactants. The dried productswere thereafter milled and passed through a number 30 mesh screen. Thereaction was conducted over a period of three hours at a reactiontemperature of about 60° C.

The six polymers prepared by this technique and the varying amounts ofmonomers and reagents are given in the table below, together with theresultant product yield for the terpolymer.

                                      TABLE 1                                     __________________________________________________________________________    Terpolymer Preparation                                                        Polymer Acrylic                                                                            2-(hydroxy propyl)     Sodium Lauryl                                                                             Yield                         No.  Units                                                                            Acid Acrylate  Acrylamide                                                                          (NH.sub.4).sub.2 S.sub.2 O.sub.8                                                     Sulfate Water                                                                             g  %                          __________________________________________________________________________    1    g* 89.0 18.0      43.0  1.00    7.00   608 157                                                                              88                              M* 1.61 0.18      0.79  5.2 × 10.sup.-3                                                                30 × 10.sup.-3                      2    g  46.0 10.0      88.0  1.00    7.00   808 149                                                                              97                              M  0.67 0.08      1.29  4.6 × 10.sup.-3                                                                24 × 10.sup.-3                      3    g  46.0 10.0      88.0  0.60   10.00   608 132                                                                              86                              M  0.84 0.10      1.63  3.4 × 10.sup.31 3                                                              43 × 10.sup.-3                      4    g  89.0 18.0      43.0  0.60   10.00   1061                                                                              105                                                                              59                              M  1.17 0.13      0.57  2.5 × 10.sup.-3                                                                31 × 10.sup.-3                      5    g  72.00                                                                              14.0      69.0  1.20    4.00   968 107                                                                              61                              M  1.03 0.11      1.00  5.4 × 10.sup.-3                                                                14 ×  10.sup.-3                     6    g  75.0 28.5      72.0  0.40    4.00   1180                                                                              157                                                                              78                              M  0.88 0.19      0.86  1.5 × 10.sup.-3                                                                11 × 10.sup.-3                      __________________________________________________________________________     *g  grams                                                                     *M  mole/kg                                                              

EXAMPLE II.

The present example was conducted to determine the effect of exposure ofvarious types and concentrations of inorganic salts to selectedterpolymers made as in Example I, above. Polymer samples No. 2 and No. 3of Example I, which contained a mole ratio of 2-to-1 ofacrylamide-to-acrylic acid, were soluble in various aqueous-saturatedsalt solutions, while those polymer samples having a smaller mole ratiowere precipitated by excess amounts of calcium ion.

The effect of the selected salt concentrations on the given polymersamples was determined by using swelling tests. The procedure for theswelling tests incorporated utilization of a 15 milliliter centrifugetube which was charged with 0.05 grams of the selected dry terpolymerwhich was powder-milled to pass through a No. 30 mesh screen. The tubealso was charged with water or the selected aqueous solutions containingvarying amounts of one of the salt samples. The total volume in the tubefor each sampled material was 13 milliliters. The tubes were stoppered,shaken vigorously to disperse the terpolymer and heated in a steam bathfor about 10 minutes. Thereafter, the tubes were cooled to roomtemperature, shaken to disperse the terpolymer, and centrifuged at 1800rpm for about 20 minutes using an International Clinical Centrifugewhich was equipped with a head having an 8.5 cm radius of gyration. Thevolume of the swollen terpolymer was measured as the difference betweenthe total volume of 13 milliliters and the volume of the supernatant.The volume of the unswollen polymer was measured utilizing isopropylalcohol as the non-solvent.

The criteria for measuring the degree of swelling for the samples wasbased upon the application of q as a criterian for measurement of thechemical potential of an electrolyte in a given solvent containing agiven amount of electrolyte which is based upon theoretical modelsdeveloped by Flory, in "Principles of Polymer Chemistry," (CornellUniversity Press, 1953) and by Hildebrand, in "Regular and RelatedSolutions" (Van Hostrand Reinhold Company, 1970) which explains theunique solubility of a polymer in various solvents.

The results given in Table 2, below, indicate that the polymer samplesNo. 2 and 3 of Example I were soluble in aqueous solutions which aresaturated with various salts, while polymer Nos. 1, 4, 5 and 6 weresomewhat precipitated by excess amounts of calcium ion. These testsindicate that these polymers have potential applications as filtrationcontrol agents exposed to high salt environments.

                  TABLE 2                                                         ______________________________________                                        DEGREES OF SWELLING, q, VERSUS IONIC                                          STRENGTH OF AQUEOUS SALT SOLUTIONS                                            Salt                                                                          conc.       Ionic     Terpolymer Number                                       mole/       Strength  1     2   3    4    5    6                              Salt    kg M    mhom/kg.sup.(1)                                                                         Degree of Swelling                                  ______________________________________                                        H.sub.2 O                                                                             0       0         gel gel gel  gel  gel  gel                          CaSO.sub.4                                                                            0.01    0.04      --  gel gel  20   gel.sup.(2)                                                                        gel                          CA(OH).sub.2                                                                          0.02    0.06      gel gel gel  gel.sup.(2)                                                                        gel.sup.(2)                                                                        gel                          KCl     4.02    4.02      7   gel gel.sup.(2)                                                                        10    8   11                           MgSO.sub.4                                                                            1.21    4.84      9   gel  --   8   gel.sup.(2)                                                                        11                           NaCl    6.00    6.00      7   gel gel.sup.(2)                                                                         8   12   12                           CaCl.sub.2                                                                            2.04    6.12      2   gel gel.sup.(2)                                                                        2     8    7                           ______________________________________                                         .sup.(1) S = 1/2Σ (MV.sup.2), V is valence of ion.                      .sup.(2) Gel formed with some swollen polymer particles suspended in          solution.                                                                

EXAMPLE III.

The filtration control effectiveness of the terpolymer samples preparedas in Example I, together with a sample of a commercially availablefiltration control agent generically described as a polyanioniccellulose (hereinafter referred to as "P.A.C.") were tested in a 22p.p.b. Wyoming Bentonite 3% sodium chloride suspension and also in a 15p.p.b. attapulgite-saturated sodium chloride suspension. The filtrationcontrol test was the API test as referred to above. The terpolymer andP.A.C. samples were sifted into barrel equivalents of muds whileshearing at moderate speed on an electric mixer after which shearing wascontinued for thirty minutes. Thereafter, the samples were hot rolled at150° F. for about 16 hours and thereafter cooled to room temperature,before flow properties and API filtrate properties were determined.

The results of this test clearly indicated that the terpolymer sampleswere effective in reducing the filtrate, when compared to a sample ofeach drilling fluid containing no filtration control additive.

The results of this test are set forth in the tables below:

                                      TABLE 3A                                    __________________________________________________________________________    FLOW AND FILTRATION CHARACTERISTICS OF 22 ppb WYOMING                         BENTONITE 3% NaCl SUSPENSION                                                  TREATED WITH EXAMPLE 1 TERPOLYMERS                                                     Fann 35 Rheology, Room Temperature                                            600 300 200 100                                                                              6  3  IG 10G                                                                              pH                                                                              API Filtrate                            __________________________________________________________________________    0.5 ppb Sample 1                                                                       52  40  34.5                                                                              28 18 17 12 42 9.0                                                                             16.0                                    1.0 ppb Sample 1                                                                       52  38  33  25 10 9  3  18 8.5                                                                             9.5                                     0.5 ppb Sample 2                                                                       44  25  19  13 3  2.5                                                                              3  11 8.2                                                                             13.0                                    1.0 ppb Sample 2                                                                       71.5                                                                              42  32  21 4  4  4  7  8.3                                                                             8.7                                     0.25 ppb Sample 3                                                                      46.5                                                                              38  35  31 28 26 28 28 8.1                                                                             19.9                                    0.5 ppb Sample 3                                                                       70  52  46  39 26 19 18 49 8.4                                                                             10.6                                    1.0 ppb Sample 3                                                                       76  63  56  46 36 29 22 69 8.3                                                                             7.2                                     0.5 ppb Sample 4                                                                       52  36  32  26 16 16 10 47 8.3                                                                             15.3                                    1.0 ppb Sample 4                                                                       50  36  32  26 13 13 8  44 8.4                                                                             9.2                                     0.25 ppb P.A.C.                                                                        55  47  43  37 24 23 21 26 8.2                                                                             18.8                                    0.5 ppb P.A.C.                                                                         75  63  57  48 29 28 28 47 8.1                                                                             11.0                                      Blank  46  39  36  32 22 22 20 15 8.3                                                                             32.5                                    __________________________________________________________________________

                                      TABLE 3B                                    __________________________________________________________________________    FLOW AND FILTRATION                                                           CHARACTERISTICS OF 15 ppb ATTAPULGITE CLAY                                    SATURATED NaCl SUSPENSION TREATED WITH TERPOLYMER SAMPLES                     EXAMPLE I.                                                                    Fann 35 Rheology, Room temperature                                            P.A.C.                                                                             600 300                                                                              200 100 6  3  IG 10G                                                                              pH                                                                              API Filtrate,ml                             __________________________________________________________________________    1 ppb                                                                              78  60 51  42  20 12 13 12 8.3                                                                             130.2                                       2 ppb                                                                              120 87 72  52  16 13 11 17 8.7                                                                             15.1                                        3 ppb                                                                              137 99 82  57  12 9  8  17 9.0                                                                             8.0                                         Sample 3                                                                      1 ppb                                                                              75  46 36.5                                                                              27  12 7  10 11 8.5                                                                             96.8                                        2 ppb                                                                              95  66 51.5                                                                              33.5                                                                              11 9  8  13 8.3                                                                             11.8                                        3 ppb                                                                              102 72 57  38  11 9  7  12 8.5                                                                             5.0                                         Sample 2                                                                      1 ppb                                                                              54  40 34  27  13 11 10 10 8.3                                                                             116.4                                       2 ppb                                                                              73  47 37  26  8.5                                                                              7  7  11 8.3                                                                             11.9                                        3 ppb                                                                              87.5                                                                              56 43.5                                                                              29  7  5.5                                                                              5   8 8.2                                                                             6.8                                         __________________________________________________________________________

From the rheological data given in each of Tables 3A and 3B, above, itmay also be observed that the addition of the terpolymer filtrationcontrol agent of the present invention did not adversely affect therheological properties of the drilling fluid, and each sample hadsatisfactory rheology properties at the given readings.

EXAMPLE IV

Nineteen terpolymer samples containing various mole percents of sodiumacrylate, 2-hydroxypropyl acrylate and acrylamide were prepared forevaluation as filtration control agents. The procedure utilized toprepare the samples was as in Example I, above. The monomers and about70 to 75 grams of water were mixed and the acrylic acid was neutralizedto about pH 7 with 5 M sodium hydroxide. The temperature of the solutionwas kept below 30° C. with the aid of an ice water bath. The monomers insolution and one-half of the initiator-amonium persulfate were addedover a one-hour period to the mixture of water, one-half of theinitiator and about 10% of the solution of monomers. The temperature ofthe reaction was maintained at about 60° C. for a total reaction time ofthree hours. The terpolymerwater mixture was broken into small samples,added to 1 liter of isopropyl alcohol, and allowed to stand for severalhours. The solvent was decanted and the residue was dried at 90° C. forabout 24 hours, followed by drying in a vacuum oven at 90° C. foranother 24 hours or to a constant weight. The samples were milled topass through a 30 mesh screen. In all cases, the product yield wasquantitative.

The various terpolymers composed of varying mole percents of sodiumacrylate, 2-hydroxypropyl acrylate and acrylamide were prepared usingthe quantities of monomers, initiator and water given in the tablebelow. The product yield was quantitative.

The reactants utilized in the preparation of the terpolymer samples areset forth in the table below.

                  TABLE 4                                                         ______________________________________                                        MONOMERS AND INITIATOR CONCENTRATIONS                                         OF TERPOLYMER PREPARATIONS                                                    Sample No.                                                                              (1)       (2)      (3)     (4)                                      ______________________________________                                         7 N*     25.6      6.3      68.1                                             M*        0.375     0.092    1.000   3.51                                     8 N       16.1      19.8     64.1                                             M         0.249     0.308    0.995   3.49                                     9 N       16.0      3.9      80.1                                             M         0.333     0.082    1.667   3.51                                     10 N      24.0      4.0      72.0                                             M         0.750     0.124    2.251   0.88                                     11 N      22.2      3.6      74.1                                             M         0.500     0.082    1.667   1.75                                     12 N      24.0      4.0      72.0                                             M         0.750     0.124    2.251   1.75                                     13 N      8.0       4.0      88.0                                             M         0.250     0.124    2.750   1.75                                     14 N      8.0       27.8     64.2                                             M         0.116     0.404    0.931   3.27                                     15 N      30        4.0      66.0                                             M         0.627     0.083    1.378   5.86                                     19 N      0         0        100.0                                            M         0         0        1.352   3.51                                     20 N      10.0      15.0     75.0                                             M         0.200     0.299    1.498   1.75                                     21 N      10.0      25.0     65.0                                             M         0.231     0.575    1.494   1.75                                     22 N      5.0       14.9     80.1                                             M         0.093     0.275    1.482   1.73                                     23 N      6.1       2.9      91.0                                             M         0.142     0.069    2.132   1.75                                     24 N      15.0      40.0     45.0                                             M         0.267     0.713    0.802   1.75                                     25 N      12.9      2.6      84.5                                             M         0.306     0.062    2.001   1.75                                     ______________________________________                                         *N mole %, M mole/kg                                                          1 sodium acrylate                                                             2 2hydroxypropyl acrylate                                                     3 acrylamide                                                                  4 ammonium persulfate                                                    

EXAMPLE V.

The present Example demonstrates the effectiveness of the presentterpolymer composition to provide effective filtration control atincreased temperatures. For this test, selected samples made as inExample I and Example IV were utilized in a mud system comprised of171/2 pounds of Wyoming Bentonite to which was added 1 p.p.b. of gypsum,2 p.p.b. sodium chloride and 2 p.p.b. of chrome lignosulfonate. Theinitial mud system was adjusted to a pH of about 9.5 with sodiumhydroxide. The selected samples of the terpolymer were added to aliquotsof the base mud in concentrations of 1/2 p.p.b. and 1 p.p.b. The sampleswere hot rolled at 150° F. for 16 hours, and were permitted to cool toroom temperature prior to rheological measurements being made and APIfiltrate being established for each sample. Thereafter, the samplecontaining 1 p.p.b. polymer was hot rolled in a 300° F. oven for aperiod of 3 hours and permitted to cool to room temperature beforerheological data and API filtrate volumes were measured. Samples of thebase mud treated with P. A. C. together with a base mud treated withanother commercially available filtration control agent identified as asodium polyacrylate, (hereinafter referred to as "S.P.A.") also wereprepared and tested, as above, for comparative purposes. This testclearly indicated that the terpolymer of the present invention is aneffective filtration control agent even after exposure to increasedtemperatures as high as about 300° F. Additionally, this test alsoindicates that the terpolymer of the present invention will not affectrheological properties of the drilling fluid. The results of this testare further illustrated in the following table:

                                      TABLE 5                                     __________________________________________________________________________    FLOW PROPERTIES AND API FILTRATE OF TERPOLYMER                                TREATED MUD BEFORE AND AFTER HOT-ROLLING AT 300° F.                                    Fann Rheology, Rm Temp.         API                           lb/bbl                                                                            Sample                                                                              Hot-Rolled                                                                          600   300 200 100 6  3  1G 10G                                                                              pH                                                                              Filtrate,                     __________________________________________________________________________                                                    ml                            1.0 13    150° F.                                                                      62    35  26  16  3  2.5                                                                              3  10 9.2                                                                             6.1                           1.0       300° F.                                                                      48    30  24  16  3  2  2  3  9.1                                                                             6.1                           1.0       150° F.                                                                      69    40  30  20  5  4  4  23 9.0                                                                             4.8                           1.0       300° F.                                                                      53    33  25  15  -- 2  4  5  9.4                                                                             6.2                           1.0       150° F.                                                                      36    20  15  9   1.5                                                                              1  2  3  --                                                                              4.8                           1.0       300° F.                                                                      71    46  38  25  4  3  3  60 9.6                                                                             5.9                           1.0  8    150° F.                                                                      66    39  29  18.5                                                                              3.5                                                                              2.5                                                                              3  7  8.9                                                                             6.2                           1.0       300° F.                                                                      64    37  27.5                                                                              17  3  2  3  3  9.5                                                                             8.6                           0.5 20    150° F.                                                                      36    21  16.5                                                                              9.5 2  1.5                                                                              1.5                                                                              2  9.2                                                                             8.2                           1.0       150° F.                                                                      44    25  19  10  2  1.5                                                                              1.5                                                                              2  9.0                                                                             6.4                           1.0       300° F.                                                                      52    30  22  13.5                                                                              2.5                                                                              2  2  2  8.3                                                                             8.1                           0.5 22    150° F.                                                                      24    13.5                                                                              10  6   1.5                                                                              1  2  2  9.0                                                                             10.4                          1.0       150° F.                                                                      42    24  17  10  2  1.5                                                                              2  3  9.0                                                                             7.2                           1.0       300° F.                                                                      57    32.5                                                                              23  13.5                                                                              2  1.5                                                                              1.5                                                                              2  8.4                                                                             10                            0.5 23    150° F.                                                                      19    11  8   5   1  1  1  1.5                                                                              9.2                                                                             7.4                           1.0       150° F.                                                                      25    14  16.5                                                                              6.5 1.5                                                                              1  1  2  9.2                                                                             5.8                           1.0       300° F.                                                                      39    23.5                                                                              18  11  2.5                                                                              2  2  3  8.6                                                                             6.5                           0.5 24    150° F.                                                                      43    25  19  12  2.5                                                                              2  2  6  8.9                                                                             8.2                           1.0       150° F.                                                                      53    31  23  14  2.5                                                                              2  2  6  8.7                                                                             8.8                           1.0       300° F.                                                                      72    43  32  19  3.5                                                                              2.5                                                                              2.5                                                                              3  8.1                                                                             10.0                          0.5 25    150° F.                                                                      19.5  11  8   5   1  1  1  11 9.1                                                                             9.0                           1.0       150° F.                                                                      26    15  11  6.5 1.5                                                                              1  1.5                                                                              2  9.3                                                                             6.4                           1.0       300° F.                                                                      42    25  18.5                                                                              11.5                                                                              2.5                                                                              2  2  2  8.4                                                                             6.5                           1.0  7    150° F.                                                                      28    15.5                                                                              11.5                                                                              7   1  1  1  2  8.9                                                                             8.6                           1.0       300° F.                                                                      26.5  14.5                                                                              10.5                                                                              6   1  1  1  1  8.1                                                                             12.6                          1.0 19    150° F.                                                                      37    33.5                                                                              18  11.5                                                                              2.5                                                                              2  3  8  9.2                                                                             37.8                          1.0       300° F.                                                                      13.5  7.5 6   3.5 1  1  1  1  8.2                                                                             38.0                          1.0       150° F.                                                                      <300  295 200 20  13 10 13 25 --                                                                              14.6                          1.0       300° F.                                                                      29    16  12  7   1.5                                                                              1  1  2  8.9                                                                             9.0                           0.25                                                                              P.A.C.                                                                              150° F.                                                                      25    14.5                                                                              11  6.5 1.5                                                                              1  2  3  8.6                                                                             10.8                          0.25      150° F.                                                                      20    12  8.5 5   1.5                                                                              1  4  2  9.2                                                                             13.6                          0.5       150° F.                                                                      35    21  15.5                                                                              10  2  2  2  7  8.9                                                                             8.4                           0.5       150° F.                                                                      26.5  15  11  7   1.5                                                                              1  3  2  8.8                                                                             10.8                          1.0       150° F.                                                                      49    30.5                                                                              24  15.5                                                                              4  3.5                                                                              4  22 8.7                                                                             5.4                           1.0       300° F.                                                                      47    29  22  14  3.5                                                                              3  4  13 9.4                                                                             9.0                           1.0       150° F.                                                                      43.5  26.5                                                                              20  13  3  2  4  12 9.2                                                                             6.6                           1.0       300° F.                                                                      31    19  14  9   2  1.5                                                                              3  7  9.3                                                                             9.7                           0.25                                                                               3    150° F.                                                                      68    45  35  23  14 14 5  63 --                                                                              12.8                          0.5       150° F.                                                                      59    37  28  17  2.5                                                                              2  2  35 8.7                                                                             7.6                           1.0       150° F.                                                                      59    37  29  19  3  2  2  8  8.7                                                                             5.2                           1.0       300° F.                                                                      61    35  25  15  2  1.5                                                                              2  2  9.3                                                                             13.0                          0.25                                                                              14    150° F.                                                                      44    29  23  15  3  2  2  55 8.8                                                                             11.0                          0.5       150° F.                                                                      73    43  33  20  5  4  4  38 8.4                                                                             9.6                           1.0       150° F.                                                                      93    58  45  29  6  5  6  49 8.6                                                                             6.8                           1.0       300° F.                                                                      82    52.5                                                                              41  27  4.5                                                                              3  4  5  9.4                                                                             14.6                          0.25                                                                              S.P.A.                                                                              150° F.                                                                      93    59  48  33.5                                                                              20 19.5                                                                             19.5                                                                             68 9.1                                                                             22.0                          0.5       150° F.                                                                      76    43  32  18  3  2  3  3  9.0                                                                             15.2                          1.0       150° F.                                                                      85    48  35  20  3  2  2  4  9.2                                                                             9.0                           1.0       300° F.                                                                      169   119 95  64  13 8.5                                                                              9  -- 9.4                                                                             20.6                          1.0       150° F.                                                                      49    28  20  12  2  1.5                                                                              1.5                                                                              2  9.6                                                                             9.0                           1.0       300° F.                                                                      69.5  41.5                                                                              31.5                                                                              18.5                                                                              2.5                                                                              1.5                                                                              1.5                                                                              2  8.8                                                                             16.4                          0.5 11    150° F.                                                                      61    38  28.5                                                                              17  3  2  2  21 9.2                                                                             6.8                           1.0       150° F.                                                                      80    60  45  30  7  6  5  47 9.2                                                                             5.4                           1.0       300° F.                                                                      78.5  45  35  21  3.5                                                                              3.5                                                                              4  8  9.0                                                                             6.6                           1.0       150° F.                                                                      77    49  38  26  4  3  3  55 9.2                                                                             4.8                           1.0       300° F.                                                                      72    42  30  18  -- 2  3  5  9.3                                                                             7.9                           0.5 12    150° F.                                                                      80    51  39  25  6  5  3  47 9.0                                                                             7.0                           1.0       150° F.                                                                      88    62  49  40  18 14 12 95 9.3                                                                             5.0                           1.0       300° F.                                                                      80    50  38  23  4  3  4  6  9.0                                                                             6.7                           0.5 15    150° F.                                                                      49    31  23  14  2  1.5                                                                              2  13 9.2                                                                             7.2                           1.0       150° F.                                                                      52    31  23  13.5                                                                              2  1.5                                                                              2  3  9.1                                                                             6.6                           1.0       300° F.                                                                      59    34  25  14.5                                                                              2  2  2  3  9.3                                                                             9.3                           0.5 10    150° F.                                                                      68    42  33  21  7  6  5  40 9.2                                                                             8.8                           1.0       150° F.                                                                      97    65  53  39  20 18 19 51 9.3                                                                             5.0                           1.0       150° F.                                                                      87    58  47  34  11 9  7  103                                                                              9.4                                                                             4.8                           1.0       300° F.                                                                      115   66  47  26  4  3  3  4  8.9                                                                             8.2                           1.0       150° F.                                                                      86    54  43  37  5  4  4  66 8.9                                                                             5.0                           1.0       300° F.                                                                      115   67  53  30  -- 3  4  4  9.0                                                                             8.6                           0.25                                                                               4    150° F.                                                                      174   146 135 111 70 61 54 55 9.2                                                                             19.8                          0.5       150° F.                                                                      88    49  35  20  3  2  2  4  9.0                                                                             14.6                          1.0       150° F.                                                                      64    38  28  17  2  1.5                                                                              2  3  9.1                                                                             7.2                           1.0       300° F.                                                                      125   86  69  47  10 7  9  17 --                                                                              19.3                          0.25                                                                               1    150° F.                                                                      92    67  55  44  33 32.5                                                                             34 70 9.1                                                                             21.6                          0.5       150° F.                                                                      57    34  25  15  2.5                                                                              2  4  6  9.2                                                                             14.8                          1.0       150° F.                                                                      44    24  18.5                                                                              10  1.5                                                                              1  2  2  9.3                                                                             7.2                           1.0       300° F.                                                                      139   96  77  53  11 8  8.5                                                                              13 9.6                                                                             18.8                          0.5  9    150° F.                                                                      26    15  11.5                                                                              7   2.5                                                                              2  3  4  9.0                                                                             7.4                           1.0       150° F.                                                                      35    20  15  9   2  1  2  3  9.1                                                                             5.2                           1.0       300° F.                                                                      61    35  26  16  3  2.5                                                                              3  3  9.0                                                                             6.1                           1.0       150° F.                                                                      39    23  17  10  2  1.5                                                                              1.5                                                                              2.5                                                                              9.2                                                                             5.4                           1.0       300° F.                                                                      52    32  24.5                                                                              15  3  2  3  9  9.2                                                                             5.6                               Base Mud                                                                            150° F.                                                                      18    9   6.5 3.5 1  1  1  1  9.3                                                                             16.4                                    300° F.                                                                      20    10.6                                                                              7   4   1  0.5                                                                              1  1  9.0                                                                             20.6                                    150° F.                                                                      21    11  7.5 4   1.5                                                                              1  1  1  9.7                                                                             20.8                                    300° F.                                                                      26.5  14  9.5 5   1  0.5                                                                              1  1  9.7                                                                             21.4                          __________________________________________________________________________

EXAMPLE VI

The present Example demonstrates the ability of the terpolymer of thepresent invention to provide effective filtration control in an aqueousdrilling fluid after being subjected to 300° F. over an extended periodof time. For this test the base mud as in Example V, was utilized. Tothe base mud was added a 1 p.p.b. treatment of the samples identified inthe Table below. The sample mud with the filtration control additiveadded thereto was first hot rolled for a period of 16 hours at atemperature of 150° F. There after, each sample was permitted to cool toroom temperature prior to measurements of rheological properties and APIfiltrate volumes. Thereafter, the initial hot rolled sample was brokeninto four separate test specimens, and each specimen was hot rolled inan oven at a temperature of 300° F. The first test specimen was hotrolled for a time period of 3 hours, the second for 5 hours, the thirdfor 7 hours and the 4th for 16 hours. After the respective time periodof hot rolling for each sample, the sample was permitted again to coolto room temperature prior to rheological measurement made and APIfiltrate being established. The same procedure was utilized to measurethe rheology and API filtrate of a blank sample of the base mud.

The results of this test clearly indicated that the terpolymer of thepresent invention provided effective filtration control of the drillingfluid sample, even after exposure to a temperature of 300° F., overextended periods of time, up to 16 hours. Additionally, the present testindicates that the filtration control agent is more effective afterexposure to a temperature of 300° F. than comparative samples ofcommercially available filtration control agents as described above.

The results of this test are further reflected in the Table below:

                                      TABLE 6                                     __________________________________________________________________________    EFFECT OF TIME AT 300° F. ON THE FLOW AND FILTRATION                   CHARACTERISTICS OF TERPOLYMER-TREATED MUD                                                    Fann 35 Reology, Rm Temp   API                                                600 300 200 100 6 3 1G                                                                              10G                                                                              pH                                                                              Filtrate                            __________________________________________________________________________     1 lb/bbl Sample 9                                                            *HR 150° F.,                                                                     16 Hrs                                                                             39  23  17  10  2 1.5                                                                             1.5                                                                             2.5                                                                              9.2                                                                             5.4                                 HR 300° F.,                                                                      3 Hrs                                                                              52  32  24.5                                                                              15  3 2 3 9  9.2                                                                             5.6                                           5 Hrs                                                                              80  51  39  25  5.5                                                                             4 4 7  9.2                                                                             7.1                                           7 Hrs                                                                              80  51  38  24  5 3.5                                                                             4 5  9.3                                                                             9.0                                           16 Hrs                                                                             86  54  41  26  5 4 4 7  9.3                                                                             9.2                                 1 lb/bbl Sample 13                                                            HR 150° F.,                                                                      16 Hrs                                                                             36  20  15  9   1.5                                                                             1 2 3  --                                                                              4.8                                 HR 300° F.,                                                                      3 Hrs                                                                              71  46  38  25  4 3 3 60 9.6                                                                             5.9                                           5 Hrs                                                                              102 65  49  31  7 5 6 15 9.3                                                                             7.0                                           7 Hrs                                                                              54  33  24.5                                                                              10.5                                                                              3.5                                                                             2.5                                                                             2.5                                                                             3  8.7                                                                             6.6                                           16 Hrs                                                                             48  29  22  14  2.5                                                                             2 2.5                                                                             3  9.0                                                                             8.0                                 1 lb/bbl P.A.C.                                                               HR 150° F.                                                                       16 Hrs                                                                             55  33.5                                                                              25.5                                                                              16.5                                                                              5 4 4 16 9.5                                                                             5.8                                 HR 300° F.,                                                                      3 Hrs                                                                              51  33  26  17  7 5.5                                                                             7 22 9.4                                                                             7.8                                           5 Hrs                                                                              40  26  20  14  4 4 5 17 9.2                                                                             10.5                                          7 Hrs                                                                              39  24  18  11.5                                                                              3 2.5                                                                             3 11 9.8                                                                             10.4                                          16 Hrs                                                                             34.5                                                                              19.5                                                                              14.5                                                                              8.5 2 1 2 4  9.2                                                                             13.6                                Base Mud                                                                      HR 150° F.,                                                                      16 Hrs                                                                             21  11  7.5 4   1.5                                                                             1 1 1  9.7                                                                             20.8                                HR 300° F.,                                                                      3 Hrs                                                                              26.5                                                                              14  9.5 5   1 0.5                                                                             1 1  9.7                                                                             21.4                                          5 Hrs                                                                              22  12  8.5 5   1.5                                                                             1 1 2  8.0                                                                             22.8                                          7 Hrs                                                                              18  9.5 7   4   1.5                                                                             1 1 1  8.0                                                                             21.6                                          16 Hrs                                                                             32  17  12  6.5 1 0.5                                                                             1 2  9.3                                                                             23.2                                1 lb/bbl S.P.A                                                                HR 150° F.,                                                                      16 Hrs                                                                             49  28  20  12  2 1.5                                                                             1.5                                                                             2  9.6                                                                             9.0                                 HR 300° F.,                                                                      3 Hrs                                                                              69.5                                                                              41.5                                                                              31.5                                                                              18.5                                                                              2.5                                                                             1.5                                                                             1.5                                                                             2  8.8                                                                             16.4                                          5 Hrs                                                                              85  53  40  24.5                                                                              3.5                                                                             2.5                                                                             2.5                                                                             -- --                                                                              20.2                                          7 Hrs                                                                              79  48.5                                                                              36.5                                                                              22  3 2 2 6  9.4                                                                             19.2                                          16 Hrs                                                                             95.5                                                                              58.5                                                                              43.5                                                                              26.5                                                                              4 3 3 6  8.4                                                                             22.8                                __________________________________________________________________________     *hot rolled                                                              

EXAMPLE VII

Tests were run and results were evaluated to determine the approximateviscosity of the terpolymer of the present invention. A 0.2% weightsolution of selected terpolymer sample was prepared by gradually adding0.60 grams, dry weight basis, of the sample to 300 grams of deionizedwater. The solution was moderately sheared for 30 minutes at roomtemperature, (about 25° C.) using an electric mixer, and thereafterallowed to stand at room temperature (about 25° C.) for 16 hours. Thesolution was then agitated with a spatula, and viscosity, incentipoises, determined at a shear rate of 511 reciprocal seconds, asdetermined by using a Fann Model 35 viscometer with bob and rotor R1B1.Centipoises reading was calculated as follows: ##EQU1##

Table VII, below, shows the results of these tests.

                  TABLE VII                                                       ______________________________________                                               Composition, Mode 9%                                                   Sample No.                                                                             A        B         C      Viscosity, cps                             ______________________________________                                        8        16.1     19.8      64.1   62                                         9        16.0     3.9       80.1   27                                         12       24.0     4.0       72.0   85                                         13       8.0      4.0       88.0   25                                         14       8.0      27.8      64.2   6                                          15       30.0     4.0       66.0   25                                         22       5.0      14.9      80.1   6                                          ______________________________________                                    

A=sodium acrylate

B=2-hydroxypropyl acrylate

C=acrylamide

Although the invention has been described in terms of specifiedembodiments which are set forth in detail, it should be understood thatthis is by way of illustration only and that the invention is notnecessarily limited thereto, since alternative embodiments and operatingtechniques will become apparent to those skilled in the art in view ofthe disclosure. Accordingly, modifications are contemplated which can bemade without departing from the spirit of the described invention.

What is claimed and desired to be secured by Letters Patent is:
 1. In amethod of drilling a well into a subterranean formation in which anaqueous drilling fluid containing clayey material is circulated into thewell, the steps of forming a filter cake on the wall of the well todecrease loss of fluid from the drilling fluid which comprises: (1)admixing with said drilling fluid an amount of at least about 0.25pounds per 42 gallon barrel of a terpolymer of: (a) a monovalent alkalimetal salt of acrylic acid; (b) an hydroxy alkyl acrylate selected fromthe class consisting of 2-hydroxyethylacrylate, 2-hydroxybutylacrylateand 2-hydroxypropylacrylate; and (c) acrylamide, a 0.2% by weightaqueous solution of said terpolymer having a viscosity at about 25° C.of from between about 6 centipoises and about 85 centipoises, asdetermined at a shear rate of 511 reciprocal seconds, the amount of saidmonovalent alkali metal salt of acrylic acid utilized in said terpolymerbeing from between about 5 mole percent and about 62 mole percent, theamount of said hydroxy alkyl acrylate utilized in said terpolymer beingfrom between about 2 mole percent and about 40 mole percent, the amountof said acrylamide utilized in said terpolymer being from between about31 mole percent and about 91 mole percent; and (2) circulating saiddrilling fluid in said well.
 2. The method of claim 1 wherein the amountof said monovalent alkali metal salt of acrylic acid is from betweenabout 10 and about 30 mole percent.
 3. The method of claim 1 wherein theamount of said hydroxy alkyl acrylate is about 4.0 mole percent.
 4. Themethod of claim 1 wherein the amount of said acrylamide utilized in saidterpolymer is about 70 mole percent.
 5. The method of claim 1 whereinthe amount of said monovalent alkali metal salt of acrylic acid is about23 mole percent; the amount of said hydroxy alkyl acrylate utilized insaid terpolymer is about 4 percent; and the amount of said acrylamideutilized in said terpolymer is about 73 mole percent.
 6. The method ofclaim 1 wherein said hydroxy alkyl acrylate is 2-hydroxypropyl acrylate.7. The method of claim 1 wherein said monovalent alkali metal salt ofacrylic acid is sodium acrylate.
 8. The method of claim 1 wherein saidmonovalent alkali metal salt of acrylic acid is sodium acrylate in anamount of about 23 mole percent; said hydroxy alkyl acrylate is2-hydroxypropyl acrylate in an amount of about 4 percent; and saidacrylamide is present in an amount of about 73 mole percent.
 9. Anaqueous drilling fluid comprising water, a clayey substance suspended insaid water, and at least about 0.25 pounds per 42 gallon barrel of aterpolymer of: (1) a monovalent alkali metal salt of acrylic acid; (2)an hydroxy alkyl acrylate selected from the class consisting of2-hydroxyethylacrylate, 2-hydroxybutylacrylate and2-hydroxypropylacrylate; and (3) acrylamide, a 0.2% by weight aqueoussolution of said terpolymer having a viscosity at about 25° C. of frombetween about 6 centipoises and about 85 centipoises, as determined at ashear rate of 511 reciprocal seconds, the amount of said monovalentalkali metal salt of acrylic acid utilized in said terpolymer being frombetween about 5 mole percent and about 62 mole percent, the amount ofsaid hydroxy alkyl acrylate utilized in said terpolymer being frombetween about 2 mole percent and about 40 mole percent, the amount ofsaid acrylamide utilized in said terpolymer being from between about 31mole percent and about 91 mole percent.
 10. The aqueous drilling fluidof claim 9 wherein the amount of said monovalent alkali metal salt ofacrylic acid is from between about 10 and about 30 mole percent.
 11. Theaqueous drilling fluid of claim 9 wherein the amount of said hydroxyalkyl acrylate utilized in said terpolymer is about 4.0 mole percent.12. The aqueous drilling fluid of claim 9 wherein the amount of saidacrylamide utilized in said terpolymer is about 70 mole percent.
 13. Theaqueous drilling fluid of claim 8 wherein the amount of said monovalentalkali metal salt of acrylic acid is about 23 mole percent; the amountof said hydroxy alkyl acrylate is about 4 percent; and the amount ofsaid acrylamide is about 73 mole percent.
 14. The aqueous drilling fluidof claim 9 wherein said hydroxy alkyl acrylate is 2-hydroxypropylacrylate.
 15. The aqueous drilling fluid of claim 9 wherein said alkalimetal salt of acrylic acid is sodium acrylate.
 16. The aqueous drillingfluid of claim 9 wherein the said monovalent alkali metal salt ofacrylic acid present in said terpolymer is sodium acrylate in an amountof about 23 mole percent; the said hydroxy alkyl acrylate is2-hydroxypropyl acrylate present in said terpolymer in an amount ofabout 4.0 percent; and said acrylamide is present in said terpolymer inan amount of about 73 mole percent.